Show number of direct calls

[stklcode]

Original feature request in WP support forums (German):
https://wordpress.org/support/topic/ohne-quelle-zur-besten-quelle-hinzufugen/

Add number of direct calls to the top lists, i.e. visits without any referrer.

This is strongly related to #118 (Internal tracking) , bacause the referrer string is currently cleared for internal hits, so it does not tell the right story at the moment.

Potential cave: Number might be too high, because the referrer might just be hidden by some redirect service or proxy.

If implemented as part of the referrer top lists, this should probably be configurable.

[patrickrobrecht]

As an absent referer URL does not mean "direct call" (e.g. due to the Referrer-Policy HTTP header or a browser addon which disables referer submission), we shouldn't call this number "direct calls", but use a describing name such as "views without referer" (and maybe give a hint for the cases that lead to the requests without referer).